Manually operated futon frames having a seat and a back linked to the seat for guided relative motion to permit adjustment between a sofa position and a bed position are well known in the art. In a common futon frame arrangement, the seat and back are pivotally connected to each other, and the back is connected to each adjacent side of the frame by respective link arms having one end pivotally connected to the back and another end pivotally connected to the associated side of the frame. In another common arrangement, pairs of follower bearings extend from the back for travel within corresponding pairs of guide channels provided in each side of the frame. With either type of arrangement, manual adjustment from a sofa position to a bed position is made by sliding the seat forward such that back is caused to follow and assume a horizontal position level with the seat. Manual adjustment from a bed position to a sofa position is carried out by pushing the seat backward at a slight downward angle to force the back into a generally vertical position. U.S. Pat. No. 5,129,114 illustrates this type of construction.
Heretofore, various attempts have been made to automate the adjustment operation of futon frames by providing a single stationary drive motor as means for indirectly driving a follower bracket connected to impart adjustment motion to the seat and back. Examples may be seen in U.S. Pat. Nos. 3,458,877; 4,563,784; and 4,937,900.
A primary challenge encountered in the design of an automatically adjustable futon frame is that a relatively large force is required to initiate backward movement of the seat to dislodge the back from its horizontal position. Prior art automatic frames have typically relied on complex multiple-bar linkages and/or brute power in the electric motor to meet this challenge. Drawbacks of a complex linkage system include added manufacturing cost, increased frame weight, and decreased reliability. Drawbacks of using a single high-powered motor include complexities in the drive train necessary to evenly transmit force to each side of the frame for smooth adjustment motion, with corresponding increase in manufacturing cost. Consequently, despite the long-recognized desirability of an automatically adjustable futon frame, as evidenced by the patents mentioned above, such item is not widely available to consumers at a reasonable price.
U.S. Pat. Nos. 5,790,993; 6,061,848; and 6,138,299, owned by the assignee of the present application and invention, disclose various configurations aimed at providing a commercially acceptable automatic futon frame. These configurations are based on a dual-motor design having a pair of motors mounted one on each opposite side of the seat to drive a respective pinion mated with an inclined rack fixed to the associated side of the frame. While these designs represented and advancement toward the goal of a commercially viable automatic futon frame, the dual-motor design proved costly, and synchronization and balance of drive forces were difficult to achieve in practice.